For many years, chromate conversion coatings have been applied to aluminum surfaces and surfaces of aluminum alloys to prevent corrosion. The compositions for forming these coatings contain, in addition to hexavalent chromium and activators such as fluorine, compounds to accelerate the reaction rate for chromate conversion coating on a volume basis. Due to the dramatic accelerating effect of potassium ferricyanide, it is the most common accelerator employed. Other accelerators enjoying limited use include molybdenum, vanadium and tungsten salts.
However, these accelerators, especially potassium ferricyanide, are toxic and they are environmentally harmful. These accelerators form toxic waste products of the conversion coating process and, due to the possible serious environmental consequences, become a serious disposal problem. The liability which may arise from utilizing these toxic chemicals as accelerators continues after disposal of the chemicals and cannot be avoided. There are other disadvantages to using such accelerators. Compounds such as potassium ferricyanide decompose during the conversion coating process, requiring repeated replenishment. Futhermore, ferricyanide compositions are also known to decompose into highly toxic hydrogen cyanide gas, thereby endangering workers. As a result, conversion coating processes have been sought which could, without the use of harmful accelerators, achieve effective coatings at a rate high enough to coat aluminum surfaces on a volume basis. Until the present invention, the results of this search have been largely negative.
The process of the present invention alleviates these problems by reducing the need for these environmentally harmful accelerators, while achieving effective chromate conversion coatings at acceptable coating rates.